Halftoning is a process of transforming a continuous-tone image into an image with a limited number of tone levels. Halftoning may be of use when reproducing or rendering the continuous-tone image with a printer or similar device that has a limited number of output states or levels. The result of the halftoning process is a digital image representation, which may be referred to as a halftone. The value of each pixel of the halftone represents one of the output levels. In the case of a color image, a separate colorant halftone may be generated for each rendered color. The various colorant halftones may then be superimposed to form a color halftone. The value of a pixel of each colorant halftone represents an output level for the corresponding colorant. For example, in subtractive four-color printing, the colorants may be cyan, magenta, yellow, and black.
For example, a binary digital printer may render an image with only two output levels. The binary printer may either print a dot, or not print a dot, at each printer-addressable pixel. The pixel values of the halftone determine whether or not a dot is printed at the each printer-addressable pixel. In the case of binary color printing, dots of several different colorants may be printed. Each colorant may be associated with a separate halftone. Each separate halftone may determine whether or not a dot of the corresponding colorant is printed at each printer-addressable pixel.
The objective of digital halftoning is to generate, using a limited number of output levels, a visual impression that is as close as possible to the original continuous-tone image. Such a visual impression is possible due to the relative insensitivity of the human visual system (HVS) to high spatial frequency patterns. Thus, the limited-level texture generated by the halftoning may be perceived by a human observer as a continuous-tone image. Similarly, a pattern of printed dots of various colors may be perceived by a human observer as a region with a single color. Limited-level texture for multiple colorants may be perceived by the observer as a continuous-tone color image.
A relatively computationally efficient technique for halftoning includes application of a point process, or screening. In screening, each pixel of the continuous-tone image is compared with a corresponding element of a selected threshold matrix, or screen. Results of the comparison may determine the value of the corresponding halftone pixel. In the case of binary digital printing, the value of a halftone pixel may be set at 1 when the value of the corresponding pixel of the continuous-tone image exceeds the corresponding screen threshold, and at 0 otherwise. In the case of color printing, separate colorant screens may be applied, each colorant screen corresponding to a colorant halftone for a different colorant. A value of a pixel of a colorant halftone may be set to 1 when a corresponding color component of the corresponding pixel of the continuous-tone image exceeds a threshold value of the corresponding colorant screen.
In the case of clustered-dot halftoning, the printer-addressable pixels at which a colorant is deposited to represent a given continuous-tone image level are clustered into compact structures, rather than being dispersed uniformly. In the case of periodic clustered-dot halftoning, the compact structures are arranged in a regular, periodic pattern. The periodicity of the pattern may be characterized by a lattice structure. In color periodic clustered-dot halftoning, the patterns for dots of the various colorants may differ from one another. Thus, deposition of each colorant may be characterized by a lattice structure specific to that colorant.